B regulatory cells promote remyelination

First we had T regulatory cells promoting remyelination. 

In this report it is the turn of the B regulatory cells to do the same thing
Tanabe S, Yamashita T. B-1a lymphocytes promote oligodendrogenesis during brain development.Nat Neurosci. 2018 Mar 5. doi: 10.1038/s41593-018-0106-4. [Epub ahead of print]

During brain development, the immune system mediates neurogenesis, gliogenesis and synapse formation. However, it remains unclear whether peripheral lymphocytes contribute to brain development. Here we identified the subtypes of lymphocytes that are present in neonatal mouse brains and investigated their functions. We found that B-1a cells, a subtype of B cells, were abundant in the neonatal mouse brain and infiltrated into the brain in a CXCL13–CXCR5-dependent manner. B-1a cells promoted the proliferation of oligodendrocyte-precursor cells (OPCs) in vitro, and depletion of B-1a cells from developing brains resulted in a reduction of numbers of OPCs and mature oligodendrocytes. Our results demonstrate that B-1a cells infiltrate into the brain and contribute to oligodendrogenesis and myelination by promoting OPC proliferation via IgM–Fc Receptor signaling.

We have heard about regulatory T cells promoting oligodendrocyte health, in this report it says that at least in mice there are B cells that accumulate in the brain and promote oligodendrocyte development.

In this study they say that B1a, B cells are present in the brain early after birth and influence oligodendrocyte development.

B1 cells are a sub-class of B cell lymphocytes that have no memory, but otherwise, B1 cells perform many of the same roles as other B cells: making antibodies against antigens and acting as antigen presenting cells

B1 cells are first produced in the foetus and most B1 cells undergo self-renewal in the periphery, unlike conventional B cells (B2 cells) that are produced after birth and replaced in the bone marrow.

Human B1 cells were found to have marker profile of CD20+CD27+CD43+CD70- and could either be CD5+ or CD5-.
Mouse B cells are more complex than human cells, B-1 B cells, in the mouse, can be further subdivided into B-1a (CD5+) and B-1b (CD5−) subtypes but the problem is are these a mouse issue without any clear human correlate. So the question is whether the have the same functions.

In the mouse, CD5 is expressed by microglia in humans they do not.  The CD5+ B1a cells have been suggested to be B regulatory cells.

Clearly it will be a growth factor(s), but which ones?